Chemical vapor deposition is a process which has developed in recent years in the semiconductor industry for manufacture of integrated and discrete semiconductor devices. In a typical manufacturing process a large (2-8 in.; 50-200 mm) wafer of silicon, germanium or similar material in extremely pure crystalline form is overlayed sequentially with numerous layers of materials which function as conductors, semiconductors or insulators. Each subsequent layer is ordered and patterned (usually by photolithographic techniques) such that the sequence of layers forms a complex array of electronic circuitry. Each device on a wafer is much smaller (generally on the order of one cm.sup.2) than the wafer. Consequently one step near the end of the manufacturing sequence is to cut the wafer along predetermined scribe lines into the many individual devices, which are commonly referred to as "chips." The devices are tested (usually before separation from the wafer) and subsequently wired into electronic devices. Such devices are key components in electronic devices ranging from toys to weapons systems to supercomputers.
All the above is well known and of course greatly simplified. The manufacturing steps of applying the individual layers of materials such as silicon, silicon dioxide, doped glasses and other layer materials are highly complex. Because the chips are so small and their circuitry so complex, almost any slight flaw or irregularity in a layer can disrupt the circuit patterns and render the chip useless. Indeed it is common for a substantial percentage of the chips on a wafer to be found upon testing to be defective for just such reasons.
CVD processes operate on the basis of two surface reaction steps. First, one or more reactive gases from which the compound or element to be deposited will be obtained are passed over the surface of the wafer under reaction conditions at which the wafer surface will catalyze the liberation of the deposit material. (In some cases a reactive gas will be introduced directly into the reactor, while in others it will be formed in situ in the gas phase in the reactor by reaction from other introduced gases.) The liberation reaction at the wafer surface may be a combination reaction in which two gases react to yield the deposit material and usually at least one by-product gas, or it may be a decomposition reaction in which a single reactive gas is decomposed to yield the deposit material and one or more by-product gases. The liberation reaction is followed by a second surface reaction in which the deposit material chemically combines with the surface of the wafer to form an integral bond and build up a layer (or "film") of deposited material. Because gas is continually flowing over the wafer surface, the types of two reactions are usually occurring simultaneously, such that previously liberated deposit material is bonding to the surface in some areas of the wafer while in others the liberation reaction is occurring. In conventional chip manufacture this process is repeated many times, interspersed with layer patterning steps, to build up the multiple layers of circuitry and insulation which are part of the final device. CVD reaction conditions and reactor parameters must be within certain narrow limits if the necessary surface reaction kinetics are to be maintained and a satisfactory yield of uniform well bonded layers of the deposited materials obtained.
It has been found that numerous problems commonly occur during CVD processes. In many CVD reactors it is difficult or impossible to get uniform gas flow throughout the reactor, so that wafers at different positions within the reactor receive different degrees of deposition. Consequently a batch of wafers from a single reactor run will not have uniform thickness of the deposited layer over all the wafers. Even over the area of a single wafer the flow irregularities can be such that the deposited layer on the wafer is nonuniform. Since the final device properties typically show a large dependence on the thickness and composition of the layers, their uniformity across a wafer and from wafer to wafer is essential.
Further, in many reactors there is substantial turbulence in the gas flow, which can cause the reactive gases to react other than at the surface of the wafers, causing deposited material to be formed in the gas stream and subsequently deposited as irregular particles on wafer surfaces. Such turbulence often involves recirculation of gas streams over wafers which have previously been coated by deposition, again causing problems such as film nonuniformities and contamination of wafer surfaces.
Even in many reactors where the flow of gas is essentially laminar (as defined by Reynolds Number), no provisions have been made to prevent the formation of recirculation cells in the gas phase. Presence of such cells causes the gases to remain in the reaction chamber for uncontrolled amounts of time. Since many reactive gases undergo preliminary reactions to form the appropriate reactive species before contacting the wafer surface, and since the gases continue to undergo these and other reactions as long as they are in the heated chamber, it is important to control the flow of the gas, both before and after it contacts the wafer, if layer uniformity and freedom from contamination are to be maintained. Recirculation makes this control very difficult.
In addition to the operating problems of CVD processes, there are a number of current limitations on CVD which prevents such processes from being readily used for certain types of film deposition. For instance, while it is sometimes possible to deposit metal alloys by CVD on a wafer substrate, this can occur only in those instances where the mixed metal-containing gases react simultaneously to liberate the metal elements under the same reaction conditions.
It has been recognized that CVD is also limited by mass transfer considerations. If sufficient reactive gas can be brought to the surface of a wafer the limiting factor is the kinetics or rate of the surface catalyzed reaction. Since the kinetics increase with temperature, control of the reaction can be obtained by temperature control, and deposition rate can be increased by increasing the temperature. In prior art reactors, however, there is a transition temperature above which the surface catalyzed deposition reaction proceeds so rapidly that the reactants cannot be brought into contact with the surface fast enough, such that mass transfer of the reactants (i.e., delivery of the reactants to the surface) becomes the limiting factor in deposition rate. Mass transfer, however, is far less dependent on temperature than is the surface reaction and therefore increase of temperature beyond the transition temperature results in much less increase in the deposition rate. Since high deposition rates of course mean faster production of chips, the present manufacturing processes are severely limited in output by the mass transfer limitations of the prior art reactors.
Also, conventional prior art reactors (other than the limited-use atmospheric pressure, conveyor belt types) do not permit one to apply more than one type of layer during a single run of the reactor. A layer of a different material can be applied only by thoroughly purging the reactor of the previous reactive gases and then introducing the second reactive gases. Essentially the prior art reactors must be run twice to form two layers.
Existing reactors are typically limited in the types of operations they can carry out, and the ranges of parameters over which they can operate. For example, many are limited to a narrow pressure range, some can operate only at atmospheric pressure, and others can operate only at low pressures (often below 10 torr). Some reactors are limited to plasma-enhanced deposition, relying on a localized plasma over the wafers to provide the energy for the chemical deposition reactions rather than utilizing the thermal energy of the wafers. Typical of these latter reactors is that disclosed in U.S. Pat. No. 3,757,733. In such reactors the gases contact the bottom of the heated wafer support plate before flowing around the plate to contact the wafers. Consequently, if heat alone, rather than a smaller amount of heat in conjunction with plasma assistance (i.e., conventional thermal CVD rather than plasma-assisted CVD) were used for the deposition reactions, there would be extensive deposition of the deposit material on the plate rather than on the wafers, with much of the deposition reaction thus having occurred before the gases even get to the wafers. In addition, in the plasma environment it is well known that electrical damage in the semiconductor device often occurs and that there is also often unwanted incorporation of hydrogen or other species into the film.
Further, many reactors (including that described in the above-mentioned U.S. Pat. No. 3,757,733) are very limited in the source chemicals they can use, and frequently cannot use the highly reactive gas mixtures, since the reactor conditions and parameters are such that the gases react before they reach the wafers or they react more on one part of the wafer than on another. Another typical example of these types of reactors is that described in U.S. Pat. No. 4,430,149, in which wafer uniformity depends on having the wafer move through a uniform environment. Thus unless the depletion rate and composition of the gas remain uniform along the travel path of the wafers through the reactor, there will be nonuniform film deposition. In the situation where two or more components come from two or more gases in the reactant mixture, not only the concentration of the gas but also its composition will vary as a function of position within the reactor, such that the wafers will receive nonuniform film composition as a function of depth through the film layer.
Yet another limitation is in the number of wafers that a reactor can handle at once; in many cases the reactor can handle only one wafer at a time.
The result of these limitations is that individual prior art reactors cannot be used for a wide variety of types of CVD operations, thus substantially increasing the cost to the user who must have several different types of reactors to be able to manufacture different types of films. The user is also often unable to optimize the chemistry and processing conditions within the range of permissible operating conditions for his particular film properties.
It would therefore be advantageous to have a CVD reactor which could rapidly and uniformly deposit a high purity layer on a wafer substrate. It would also be advantageous if such reactor were capable of depositing a plurality of materials on the wafer in a single run, particularly where such individual materials are not otherwise capable of being combined in CVD processing. Finally, it would be advantageous if the reactor were capable of depositing a number of different film types over a wide range of operating conditions with little or no modification from one type of operation to another.